Because individual stocks can be rigged in either direction (for or against your position), but at the level of sector or index funds all of that activity balances out.
Sadly, insider trading is pervasive on wall street. You don't read about the guys that don't get caught, and there are a lot of them. Remember, if you can't spot the sucker at the table, you're the sucker. That's why I keep my money far away from the stock market except for index funds.